We study the morphology of the central region of a sample of Active
Galactic Nuclei (AGN) and a “control” sample of normal galaxies using
archival observations of the WFPC2 instrument onboard the Hubble
Space Telescope (HST). We use the ellipse fitting technique in order to
get a good description of the inner “smooth” light distribution of the
galaxy. We then divide the observed galaxy image by the artificial image
from the fitted ellipses in order to detect morphological signatures in
the central region around the nucleus of the galaxy. We perform
quantitative comparisons of different subgroups of our sample of galaxies
(according to the Hubble type and the nuclear activity of the galaxies) by
calculating the average amplitude of the structures that are revealed with
the ellipse fitting technique. Our main conclusions are as follows:
1) All AGNs show significant structure in their inner 100 pc and 1 kpc
regions whose amplitude is similar in all of them, independent of the
Hubble type of the host galaxy.
2) When considering early-type galaxies, non-AGN galaxies show no
structure at all, contrary to what we find for AGN.
3) When considering late-type galaxies, both AGN and non-AGN galaxies show
significant structure in their central region.
Our results are consistent with the hypothesis that all early-type
galaxies host a supermassive black hole, but only those that have enough
material in the central regions to fuel it show an active nucleus. The
situation is more complicated in late-type galaxies. Either not all of
them host a central black hole, or, in some of them, the material
inside the innermost 100 pc region is not transported to the scales of the
central engine for some reason, or the large amount of gas and dust hides
the active nucleus from our sight.

Key words: galaxies: active / galaxies: Seyfert / galaxies: structure

*

Based on observations made with the NASA/ESA Hubble Space
Telescope, obtained from the data archive at the Space Telescope
Science Institute. STScI is operated by the Association of Universities
for Research in Astronomy, Inc., under the NASA contract NAS5–26555.

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